I really am sorry to hear that this is happening to both you and your daughter, OP. I’m not gonna cast in a vote for the verdict since I’ll feel like an asshole myself for calling a terminally ill person an asshole, but instead I’ll agree with the majority of what the comments have already said and divide the money up through a trust. Once each granddaughter/grandson reaches a certain age, they get their share of the money. You can also give your son his share right off the bat.

I understand where you’re coming from with not wanting to leave 500k to a woman in her 20s. You want her to mature enough and experience life without becoming a spoiled brat. It’s, again, where the trust comes in. Decide the amount you want to give each grandchild evenly and decide when you think they’ll be mature enough to get it. That way, your granddaughter won’t be regretting a splurge of spending when she doesn’t know better.

Also, I’d talk about things with your son and daughter. Try to work things out together regarding the will, you know? It’s best to decide what to do as a family... It’s also good to spend time with them before yours is up.